The present invention relates to digital radio communication systems. More particularly, and not by way of limitation, the present invention is directed to a transmitter and method for transmitting a sequence of transmitted symbols in a digital communication system utilizing soft pilot symbols.
In digital communication systems, the receiver must estimate some parameters in order to correctly demodulate the transmitted data. The receiver may also need to estimate a measure of signal quality to feed back to the transmitter. The estimation of parameters/signal quality generally falls into three categories:
(1). Blind estimation. Generally this approach relies on some signal or channel property/characteristic that is known a priori or learned in a slow manner (for example, second-order statistics). The biggest problem with blind estimation is performance. Blind estimation generally underperforms other approaches by a significant margin. Also, blind estimation algorithms may be more complex.
(2). Pilot-aided. This approach includes known (i.e., pilot) symbols in the transmitted signal. Pilot symbols can be embedded in the data sequence (for example, the midamble of GSM) or allocated a separate resource such as the pilot code in WCDMA, so long as the pilot symbols experience the same effective fading channel as the data. The pilot-aided approach generally offers the best performance. However, pilot symbols consume resources that might otherwise be devoted to transmitting useful data. Typically there is a tradeoff between having sufficient pilots for good estimation and maximizing data throughput.
(3). Data-aided. This approach uses demodulated data symbols as “extra” pilot symbols. Generally this approach is used in conjunction with either blind estimation or the pilot-aided approach. There are two problems associated with the data-aided approach. First, blind estimation or pilot-aided estimation (or both) is typically required as a first receiver step. Therefore, data-aided approaches require extra receiver complexity. Second, data-aided approaches can degrade receiver performance due to the effect of errors in demodulating data. In data-aided approaches, the demodulated data symbols are assumed to be correct and are used as additional pilot symbols. However, if the data symbols are incorrect, the parameter/signal quality estimation algorithms can produce incorrect results. The effects of incorrect symbol decision(s) can persist for more than one estimation interval, so data-aided approaches may need special mechanisms to avoid the effect of error propagation.
The data-aided approach has been utilized in a number of existing communication systems. For example, in Wideband Code Division Multiple Access (WCDMA) systems, the control channel on the uplink is demodulated/decoded, and the symbol decisions are used as effective pilots. This has also been proposed for the WCDMA control channel on the downlink. In the Digital Advanced Mobile Phone System (D-AMPS), the channel is first estimated over a synchronization word and then tracked over data during equalization. In the equalizer, early temporary unreliable decisions are fed to the tracker, and delayed better decisions are fed to the decoder. Also in D-AMPS and GSM, multi-pass (turbo) demodulation/decoding uses decoded/re-encoded symbols as effective pilots in a second pass.